It will be readily apparant that the steepness of the slope of the upper screen surface will affect both the speed and the precision of the sieving process, since a more steeply tilted screen will cause the coarser particles to pass more rapidly across the screen surface, thus increasing the rate at which material may be fed to the screen, but simultaneously the particles will be afforded less opportunity to pass through the screen apertures, and therefore that fraction of the feed material which passes over the upper screen surface will contain some particles which otherwise could have passed downwardly through the screen apertures. It is therefore normal practice with such machines to make provision for adjustment of the screen slope angle, in order to enable the operative to optimise the rate of throughput and the precision of the sieving process with respect to each other.
A typical tiltable vibratory screen has a nest of two or more screens arranged in such a manner that material which passes through the uppermost and coarsest screen then falls under gravity upon the surface of a subsequent and finer screen. Therefore, it is necessary to provide a troughed belt conveyor or other means to elevate the unscreened material to a height necessary to feed the material so that it falls continuously upon the uppermost screen adjacent to its uppermost edge, whilst simultaneously providing adequate space or headroom beneath the lowermost edge of the lowermost screen, to accommodate suitable means for the separate collection and removal of the various separated fractions.
It will be apparent that the positions of the uppermost and/or the lowermost edges of the screens with conventional apparatus will be altered each time the angle of tilt of the screen assembly is adjusted, and hence it becomes necessary to adjust the relative positions of the feed to the screen assembly and the means for collecting the separated fractions, usually in the form of discharge collection chutes.